A gas discharge lamp, such as a fluorescent lamp, typically utilizes a ballast circuit to convert an a.c. line voltage to a high frequency a.c. voltage which is impressed across a resonant load circuit containing the gas discharge lamp. The resonant load circuit includes a resonant inductor and a resonant capacitor for determining the frequency of resonance of current in the resonant load circuit. The ballast circuit typically includes a series half-bridge d.c.-to-a.c. converter having a pair of power switches that alternately connect one end of the resonant load circuit to a d.c. bus voltage and then to a ground, thereby impressing the mentioned a.c. voltage across the resonant load circuit. Typically, gate-drive circuitry is provided to control the switches of the converter in a regenerative, or self-resonant, manner.
The above cross-referenced patent applications relate to the implementation of regenerative gate-drive circuitry, which beneficially can be implemented in state form. As such, the circuitry does not require the use of a magnetic transformer, as do many prior art circuits, and many of its components can be implemented in an integrated circuit. However, it would be desirable to improve upon the gate-drive circuitry of the above crossreferenced applications, and to include a cathode preheat function, whereby the cathodes of the lamp are heated for a predetermined period of time prior to ignition of the lamp. During the cathode preheat period, it would be desirable to maintain a low lamp voltage to prevent ignition of the lamp before the cathodes are sufficiently heated.